UK palaeontologists led by Professor Jennifer Clack from the University of Cambridge discovered the fossils over a 23-year period. After searching in Devonian-age rocks in Scotland for over 20 years, Clack's colleague Stanley Wood and Tim Smithson finally chanced upon the fossils.

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They include animals both with and without backbones — vertebrates and invertebrates — which lived either in water or on the land between 360 and 345 million years ago. This 15-million-year stretch runs from the end of the Devonian to the early Carboniferous periods.

The findings help bridge a puzzling gap in the fossil record between Devonian tetrapods, which were primitive, four-legged, aquatic animals, and the more complex, mostly land-dwelling post-Devonian tetrapods. These more-developed terrestrial animals are essentially our distant ancestors.

'The break has been frustrating, because you wouldn't expect evolution to jump from simple aquatic creatures to complex, terrestrial animals without something in between,' explains Clack.

The gap, named Romer's Gap after the palaeontologist who discovered it, was an even longer 30 million years when Romer first identified it. Although the gap is slowly closing, the break that covers the key 15 million years, when our ancestors left the water to live as land-dwellers, has existed for decades. Until now, palaeontologists had just two fossilised examples of animals from this interval — one from Scotland, the other from Nova Scotia.